1. Field of the Invention
The present invention relates to a crystalline polymer microporous membrane, and a filtration filter using such crystalline polymer microporous membrane.
2. Description of the Related Art
Microporous membranes have been known for long and widely used for filtration filters, etc. As such microporous membranes, there are, for example, a microporous membrane using cellulose ester as a material thereof (see U.S. Pat. Nos. 1,421,341, 3,133,132, and 2,944,017, Japanese Patent Application Publication (JP-B) No. 48-40050), a microporous membrane using aliphatic polyamide as a material thereof (see U.S. Pat. Nos. 2,783,894, 3,408,315, 4,340,479, 4,340,480, and 4,450,126, German Patent No. 3,138,525, and Japanese Patent Application Laid-Open (JP-A) No. 58-37842), a microporous membrane using polyfluorocarbon as a material thereof (see U.S. Pat. Nos. 4,196,070, and 4,340,482, and JP-A Nos. 55-99934 and 58-91732), a microporous membrane using polypropylene as a material thereof (see West German Patent No. 3,003,400), and the like.
These microporous membranes are used for filtration and sterilization of washing water for use in the electronics industries, water for medical use, water for pharmaceutical production processes and water for use in the food industry. In recent years, the applications of and amount for using microporous membranes have increased, and microporous membranes have attracted great attention because of their high reliability in trapping particles. Among them, microporous membranes made of crystalline polymers are superior in chemical resistance, and in particular, microporous membranes produced by using polytetrafluoroethylene (PTEF) as a raw material are superior in both heat resistance and chemical resistance. Therefore, demands for such microporous membranes have been rapidly growing.
These microporous membranes are used for filtration and sterilization of washing water for use in the electronics industries, water for medical use, water for pharmaceutical production processes and water for use in the food industry. In recent years, the applications of and amount for using microporous membranes have increased, and microporous membranes have attracted great attention because of their high reliability in trapping particles. Among them, microporous membranes made of crystalline polymers are superior in chemical resistance, and in particular, microporous membranes produced by using polytetrafluoroethylene (may also referred to as “PTEF” hereinafter) as a raw material are superior in both heat resistance and chemical resistance. Therefore, demands for such microporous membranes have been rapidly growing.
As for such crystalline polymer microporous membrane, there is a proposal of a porous membrane produced by rolling a multi-layer paste formed of polymers each having a different fusion thermal peak of crystals (see JP-A No. 03-179038). Since pores whose diameters are continuously changes along with the thickness direction are not formed in this porous membrane, the membrane cannot efficiently capture fine particles, and has short service life as a filter.
In order to efficiently capture fine particles, moreover, there is a proposal of a membrane, which is formed by laminating layers each formed of a polymer of a different molecular weight, and in each layer of which pores are formed to have a diameter that continuously changes with respect to the thickness direction of the layer (see JP-A No. 2009-61363). However, in this crystalline polymer microporous membrane, a thickness of a dense layer formed of a low molecular weight crystalline polymer, it is difficult to satisfy all of the required properties for the membrane, such as high flow rate, no clogging, long service life, and high strength, at the desirable balance.
Accordingly, there is currently strong demands for a crystalline polymer microporous membrane and a filtration filter using such membrane, which are capable of efficiently capturing fine particles, have high filtration rate, do not cause clogging, has long service life, and have high durability.